Conversion of solar energy to electrical and/or heat energy

ABSTRACT

A parabolic primary mirror ( 10 ) has a concave specular surface ( 12 ) that is constructed and positioned to receive solar energy and focus it towards a focal point. A secondary mirror ( 14 ) having a convex specular surface ( 16 ) is constructed and positioned to receive focused solar energy from the primary mirror and focus it onto an annular receiver ( 18 ). The annular receiver ( 18 ) may include an annular array of optical elements ( 100 ) constructed to receive solar energy from the secondary specular surface ( 14 ) and focus it onto a ring of discrete areas. A ring of solar-to-electrical conversion units are positioned on the ring of discrete areas.

TECHNICAL FIELD

Improvements in the conversion of solar energy to electrical and/or heatenergy are described. More particularly, a system of mirrors andlenses/prisms for economically collecting solar energy and converting itto electrical and/or heat energy is described.

BACKGROUND

Solar energy has been a desirable energy source for over thirty years.However, cost has always been an obstacle to its widespread use. Themost familiar solar energy systems comprise an array of solar cells thatcover enough area, or intercepts enough incident sunlight. to yield thedesired amount of electrical power at relatively low conversionefficiencies of ten to fifteen percent (10%-15%). This approach requireslarge areas of expensive semi-conductor solar cells. To date, thesesystems have been uncompetitive without cost subsidies of some sort. Ingeneral, the prohibitive cost of solar energy systems has been primarydue to the cost and the quantities required of the semi-conductorconversion devices called solar cells. There have been severalapproaches to alleviating the cost issue. One approach is to fabricatethin-film solar cells that use only a minimal amount of semi-conductormaterial. Unfortunately, this approach generates still lowerefficiencies, six to eight percent (6%-8%) and the materials have provento be problematic. A second approach has been used various opticaldevices such as fresnel lenses or mirrors to concentrate the solarenergy to higher intensity and then convert it using a smaller area ofthe expensive solar cells. All of these approaches have been, and arestill being pursued. None to date have resulted in economical solarenergy generation without some sort of financial incentives beingoffered by the utilities or by government agencies. There is a need fora more economical way of collecting solar energy and converting it intoelectrical and/or thermal energy.

BRIEF SUMMARY

The solar energy collection system according to the various embodimentscomprises a primary mirror and a secondary mirror. The primary mirrorhas a concave specular surface constructed and positioned to receivesolar energy and focus it towards a focal point. The secondary mirrorhas a convex specular surface constructed and positioned to receivefocused solar energy from the primary mirror and refocus it onto anannular receiver.

In an embodiment, the annular receiver includes an annular array ofoptical elements constructed to focus the solar energy received from thesecondary specular surface onto a ring of discrete areas. In the variousembodiments, a ring of solar-to-electrical conversion units arepositioned on the ring of discrete areas.

In the various embodiments, the concave specular surface of the primarymirror is substantially parabolic. The convex specular surface of thesecondary mirror is a hyperbolic surface modified to refocus the solarenergy onto the annular receiver.

In the various embodiments, the annular receiver comprises a pattern oflenses/prisms arranged to further concentrate the solar energy anddeliver it onto an annular array of photovoltaic cells.

The various embodiments also includes methods of making the primary andsecondary mirrors. It also relates to a relationship between thesecondary mirror and an optical concentrator, and between the opticalconcentrator and a system of photovoltaic cells. The photovoltaic cellsserve a dual function in the system. They absorb the concentratedsunlight and convert a portion of it to electricity and a portion toheat, or thermal energy. Thus, it serves as both an electrical generatorand a heat generator. In order to accomplish these two rolesefficiently, the photovoltaic cells are fabricated for semiconductormaterials with sufficiently wide band gap to maintain the efficientelectric conversion at relatively high temperatures. In general, thewider the band gap of the semiconductor material, the less thephotovoltaic cells efficiency will be degraded with rising temperature.A tradeoff is required for the application being considered depending onthe relative importance of electricity production and heat production.

The various embodiments provide a mirror that is composed of a thinmetal body having a curved specular surface, comprising a polymer layeron said metal body surface, a reflective metal layer on the polymerlayer, and a thin glass layer on the metal layer. This construction canbe used for both the primary mirror and the secondary mirror.

In the various embodiments, the thin-metal body of the mirror is formedfrom sheet aluminum alloy. A particularly suitable alloy is aluminumalloy 6061 that has been hardened to a T-6 condition. The thin metalbody is formed into a desired shape and then is rotated while thepolymer layer, the reflective metal layer and the thin glass layer aresuccessively applied to it.

In the various embodiments, the specular surface of the secondary mirroris a convex surface that has been shaped to cause it to reflect andfocus light/heat energy received by it onto an annular focus area.

In the various embodiments, a system lends itself well to wide band gapphotovoltaic cells having both single and multiple tandem junctions. Todate, the cost of photovoltaic cells made from wide band gap materialsand in multi-junction configurations has precluded use in terrestrialapplications. The concentrator system produces a very high lightintensity and allows the use of a small, economical area of photovoltaiccells.

The various embodiments includes a unique design of high intensityphotovoltaic cells. These cells have unique, long and narrow activeareas that are optimum for two reasons. Firstly, the cell patterncorresponds to the illumination pattern provided by the tertiaryconcentrator lenses. Secondly, the cell pattern provides a very shortpath length for conducting the photo-generated current off the cells.Photovoltaic cells under light concentration operate very large currentsat low voltage. Therefore, any series resistance in the cell would dropthe voltage and, in turn, the efficiency of the cells. The current fromthe high intensity cells is collected and conducted off of the cells bymeans of a pattern of electrically conducting metal grids overlaying theactive areas of the cells. The series resistance in the grids isproportional to the length of the grids. For this reason, the largenarrow cell design with its electrical bus bar running parallel to thelong dimension of the cells permits the necessary short conductor grids.As will hereinafter be described, the various embodiments includes aconstruction of the photovoltaic cells and the pattern of such cells.

In the various embodiments, a solar energy conversion system convertssolar to thermal energy in the form of hot water at useful temperatureswhile simultaneously converting solar power to electrical power at highefficiency. In the system, the concentrated solar energy is firstabsorbed by the photovoltaic cells. The photovoltaic cells convert aportion of the absorbed energy to electricity because the photovoltaiccells are made from wide band gap semiconductor materials, they canmaintain high efficiency even at elevated temperatures.

In the various embodiments, a sensor and control system consisting of asun sensor may be provided to supply sun position signals to amicrocomputer that processes information and sends control signals togear motors that drive the concentrators and hold them locked onto thesun to an accuracy of +/−0.1 degrees. The micro computer system furtherserves to shut the system down at night and position the primary mirrorsto face the ground, wake the system up in the morning and acquire thesun, monitor the photovoltaic cell temperatures and drive theconcentrators off sun if the cells overheat, monitor wind speed androtate the concentrator mirrors to face down (edge-on to the wind) ifwind speed exceeds a threshold amount.

Other objects, advantages and features will become apparent from thedescription set forth below, from the drawings, and from the principlesthat are embodied in the specific structures that are illustrated anddescribed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Like reference numerals and letters refer to like parts throughout theseveral views of the drawing, and:

FIG. 1 is a sectional schematic view of an embodiment;

FIG. 2 is an enlarged scale fragmentary view of an edge portion of FIG.1, such view showing a thin metal mirror body, a polymer layer on thebody, a reflective metal layer on the epoxy polymer layer, and a thinglass layer on the metal layer;

FIG. 3 is a block diagram of a total system embodying solar collection,electrical, thermal and control components;

FIG. 4 is a graph of temperature vs. time presenting time/temperaturecharacteristics for quenching 6061 aluminum alloy;

FIG. 5 is a graph of yield strength vs. cooling rate showing relativestrengths of 6061 aluminum alloy as a function of quenching parameters;

FIG. 6 is a diagram of the forces involved with spinning a smoothpolymer coating onto the parabolic surface of the primary mirror;

FIG. 7 is a schematic diagram of a vacuum deposition chamber andion-assisted deposition elements;

FIG. 8 is a diagram of a hyperbola showing its geometrical axis andshowing a real focal point at one end of the axis and an imaginary focalpoint at the opposite end of the axis;

FIG. 9 is a view like FIG. 8, but showing the hyperbola and itsgeometric axis tilted by an angle x from the original position of thegeometric axis, and showing a portion of the hyperbola extending from apoint a to a point b;

FIG. 10 is a view combining FIG. 8 and FIG. 9 and showing the real focalpoint of the rotated hyperbola section a-b transversing a circular path;

FIG. 11 is a view of a three dimensional shape formed by rotation of thehyperbolic section a-b about a new geometrical axis c-c;

FIG. 12 is a diagram of the forces involved with spinning a smoothpolymer coating onto the hyperbolic surface of the secondary mirror;

FIG. 13 is a cross sectional view of a lens/prism assembly spaced fromthe secondary mirror;

FIG. 14 is a pictorial view of a circular array of glass lens/prismelements, with one of the elements shown moved up from its position inthe array;

FIG. 15 is an enlarged side view of one of the lens/prism elements;

FIG. 16 is an end view of the lens/prism element of FIG. 14;

FIG. 17 is a top view of the lens/prism element shown by FIGS. 14 and15;

FIG. 18 is a schematical view of the electrical components of a systemthat includes four of the solar collection systems shown by FIG. 1;

FIG. 19 is a plan view of a unique photovoltaic cell that operatesefficiently at very-high light intensities;

FIG. 20 is an exploded pictorial view of a circular array of thephotovoltaic cells shown by FIG. 19 together with cell interconnects,protective diodes, a circuit etched in a copper laminate, a ceramicsubstrate and another copper laminate below the ceramic substrate;

FIG. 21 is an electrical schematic of the photovoltaic cell array shownin FIG. 20;

FIG. 22 is a schematic diagram of the thermal system and itsrelationship to the other components of the overall system;

FIG. 23 is a graph showing a typical reflectance curve for glass as afunction of the angle of incidence of incoming light;

FIG. 24 is a graph showing the signal produced in a glass covered solarcell as a function of angle;

FIG. 25 is a top plan view of an array of sensors which is verysensitive to small angular changes;

FIG. 26 is a side view of the array of sensors shown by FIG. 25;

FIG. 27 is a cross sectional view of an entire sun sensor system,showing a rear sun sensor assembly, a coarse sensor assembly and a finesensor assembly; and

FIG. 28 is a graph showing a typical signal generated by the sun sensoras a function of error angle.

DETAILED DESCRIPTION

FIG. 1 is a schematic view of the optical components of the illustratedembodiment. It comprises a primary mirror 10, a secondary mirror 14 andan optical concentrator 18. Primary mirror 10 is a concave/convex dishthat has a specular surface 12 on its concave side. Secondary mirror 14is a concave/convex dish having a specular surface 16 on its convexside. The convex side 16 of mirror 14 confronts the concave side 12 ofthe mirror 10. The concave specular surface 12 is preferably parabolicand the specular surface 16 is generally hyperbolic. The substantiallyparabolic surface 12 is shaped to focus axially collimated sun rays to afocus point co-planar to its rim 20. This focal point is coincident withthe virtual focal point of a generally hyperbolic specular surface 16 ofthe secondary mirror 14. Specular surface 16 is constructed andpositioned to concentrate the solar energy that it receives from theprimary mirror surface 12 and concentrate it onto an annular area at oneend of an optical concentrator 18. As will hereinafter be described indetail, concentrator 18 is composed of an annular array of lenses/prisms100 that further concentrate the solar energy onto a ring of discreteareas. Photovoltaic cells PV are positioned on the discrete areas forabsorbing the concentrated solar area and converting it to electricityand heat.

In FIG. 3, the solar collection portion of the system is designated SC.The electrical portion of the system is designated ES. The thermalportion of the system is designated TS. The control portion of thesystem is designated CS.

Preferably, the primary mirror 10 is constructed from a sheet ofaluminum alloy that has been formed to give it a substantiallyparabolic, convex/concave shape and a circular rim 20 composed of radialand cylindrical flanges 22, 24. As will hereinafter be described in moredetail, an epoxy polymer layer 26 is deposited on the concave surface 12(FIG. 2). This is followed by a vapor deposition of a thin film 28 ofreflective metal on the polymer layer 26. Then, to protect the metallayer 28 from corrosion and oxidation, a thin glass layer 30 isdeposited on the metal layer 28. This protects the metal layer 28 fromweather, abrasion and cleaning.

The secondary mirror 14 is preferably also formed by a thin sheet of analuminum alloy that is shaped to provide it with a modified hyperbolicspecular surface. As with the primary specular surface 12, the secondaryspecular surface 16 is provided with a layer of polymer over the formedsheet of aluminum. Then, a reflective metal layer is applied to thepolymer layer and a thin glass layer is applied to the metal layer.

The primary and secondary mirrors 10, 14 are supported by a common frameF to which the primary mirror 10 is connected by plates 86, 88 and aseries of fasteners (not shown). This frame F includes an axiallyextending post P which is coincident with a common center line axis ofthe two mirrors 12, 16. The annular array of lenses/prisms 100 forming apart of the concentrator 18 surround the post P. As will hereinafter beexplained in greater detail, the modified hyperbolic surface 16 isconstructed and positioned to focus the solar energy onto the annularring of lenses/prisms that are a part of concentrator 18. The secondarymirror 14 may include a housing on its concave side constructed toreceive a cooling fluid.

The primary mirror 10 may be formed by hot blow forming a heated sheetof aluminum into a die that is precisely machined to the desiredparabolic shape. The hot blow forming process is so named because ituses gas pressure to force the heated sheet into conformance with thedie. Forming the sheet at high temperature allows the use ofoff-the-shelf rolled aluminum sheet for substrate stock. The highforming temperature lowers the tensile strength of the material, so thatthe internal stresses that cause spring back from the die are minimized.The lowered tensile strength also minimizes variations in spring backdue to differences between batches of materials or in materials fromdifferent vendors. The gas pressure presses equally on all parts of thesheet ensuring that all areas of the material are precisely conformed tothe die and is left very precisely conformed to the shape of the formingdie. After the forming process, the formed part is in a soft (“T-0”)annealed condition. The soft condition is undesirable and can be avoidedby the selection of a suitable alloy and appropriate forming conditionsso that age hardening, or tempering, can occur in the part after it isremoved from the die and cooled.

Aluminum alloy 6061 is a suitable material to utilize the age hardeningprocess. Age hardening the formed aluminum part to its T-6 conditionmakes it about five times harder, or stiffer, than it was in the T-O(soft annealed) condition. Aluminum alloy 6061 is an excellent choicefor age hardening because of its relatively slow impurity precipitationrate during cooling. FIG. 4 illustrates the solid state properties ofthe material as a function of average cooling rate between the criticaltemperatures of 400 and 290° C. Generally, soaking the primaryconcentrator (mirror) at temperature for fifteen (15) minutes issufficient to set up the age hardening conditions. Once the requiredsolid solution of impurities is obtained, cooling, or quenching, thealuminum rapidly enough, prevents the impurities from precipitating outof solution and yields a supersaturated alloy. After the part is cooled,the impurities slowly precipitate to relieve the supersaturatedcondition. This results in a non-equilibrium, unstable, microstructurethat will, over a period of hours to weeks (depending on thetemperature) decompose into a multiphase system with the precipitatedimpurities creating lattice strains and distortions which harden thealuminum to the rigid and strong T-6 condition. FIG. 5 illustrates therelative strengths of the 6061 alloy as a function of average coolingrate between the critical temperatures of 400 and 290° C.

The conditions for heating and cooling are established for the 6061aluminum alloy to be both precisely formed without spring back and toinduce age hardening. The conditions for both the forming and quenchingmust be achieved in the forming die. This is achieved by thermallyisolating the aluminum sheet blank in the forming chamber and usingradiant heating to raise the aluminum sheet to the 536 C solutiontemperature. The part is then rapidly blown into a steel die that ismaintained at a temperature of 225° C. that is below the criticaltemperature for precipitation in the aluminum alloy. The aluminum sheetrapidly gives up its heat to the die and is cooled to the point requiredfor age hardening.

The forming machine performs a number of functions in the process ofheating, forming and then quenching the primary mirror. It thermallyisolates the dish blank to allow heating. It forms a gas-tight sealbetween the blank and the die. It forms a stiffening ring around theouter edge of the disk, forcing the blank material into the die andholds it in place as the die cools the formed dish to set-up theage-hardening condition. The formed primary mirror is removed from theforming machine and stored for age hardening before a specular surfaceis formed.

A specular surface is formed on the concave side of the formed aluminumbody 10. This is done by spinning the thin metal body 10 and applying toit a layer of polymer. A liquid polymer is placed at the center of thedish which is then spun about its geometrical axis to a rotational speedsuch that centrifugal force causes the liquid to flow outwardly andupwardly along the surface of the dish 10 to its outer rim 20. When theentire concave surface 12 of the primary mirror 10 is covered with afilm of the liquid polymer, the dish spin rate is adjusted so thatcentrifugal forces exactly cancel gravitational forces. At this point,there are no net forces on the liquid so that it becomes a stationaryparabolic sheet 26 with its surface tension smoothing it to specularity.The condition of no net forces on the liquid is shown in FIG. 6. Therequired spin conditions for the fluid can be calculated as shown below.

FIG. 6 illustrates the conditions to be met relative to the curvature ofthe parabolic dish. The origin is assumed to be at the center of thedish, the axis if normal to the origin, the radius, R_(max) is 28 inchesand the rim height, H_(max) is 14 inches. Thus, the parabolic curvatureis described byh=1/56r ²  (1)anddh/dr=r/28=tan θ  (2)where h is the height above the origin and r is the radial distance fromthe origin. Then the tangential gravity (centripetal) force isF_(gt)=F_(g) sin θ=mg sin θ  (3)where F_(gt) is the tangential component of the gravitational force,F_(g) is the total gravitational force, θ is angle of the tangent to thecurve with respect to the horizontal, m is the incremental mass of thefluid and g is the gravitational constant. Then the tangentialcentrifugal force isF_(ct)=mrw² cos θ  (4)where F_(ct) is the centrifugal force and w is the angular velocity ofthe spinning dish.

By equating equations (3) and (4) and solving for w we getw=√8/28 radians or rpm=60/2π√g/28  (5)When this rotational speed condition is met then the fluid remainsstationary at all points on the substrate surface and has no tendency torun. By the same token, since the derivation is based on the startingpremise of a parabolic curvature then the fluid seeks to fulfill theparabolic curve and can actually correct for small forming flaws in thestarting substrate.

The liquid polymer also has certain requirements that must be fulfilledto properly function in the above application. The polymer propertyrequirements are (1) the viscosity must be sufficiently low to alloweasy flow over the surface of the dish, (2) the working time must allowvacuum out-gassing for bubble removal as well as spinning the materialonto the dish, (3) it permits a thermally activated cure, (4) curedmaterial must be vacuum compatible, (5) cured material must withstandthe heat of vacuum vapor disposition of metal and dielectric layers, (6)it must support these deposited layers without wrinkling and (7) thecured material must be tolerate of humidity and thermal cycle duringnormal service.

The integrated manufacturing process produces a primary mirror 10 thatis durable, economical, precisely formed, and optically superior. Thekey steps of the manufacturing process are forming the shape, smoothingthe surface and deposition of the highly reflective metal and protectiveglass layers. The reflective metal layer is deposited onto the curedpolymer, smoothing layer during a high-vacuum deposition process. Duringa single vacuum pump-down process, both the highly reflecting metalsurface and the protective glass layer are deposited. A critical aspectof the deposition of the metal and glass layer is to deliver additionalenergy to the layers as they are being deposited on the mirrorsubstrate. The added energy is delivered to the surface by ionizing aportion of the material being deposited and then accelerating these ionstowards a surface where the films are being grown. These ions releasetheir kinetic energy to the growing film allowing lateral movement ofdeposited material to density the growing film, minimize pinholeformation, enhance film adhesion and form a bulk-like layer.

Deposition of the metal and glass layers is accomplished in ahigh-vacuum chamber, schematically shown by FIG. 7. The process beginswith the smoothed primary mirror 10 being introduced into a vacuumchamber 70 and sufficient gases being removed from the chamber 70 toyield a pressure of 4.0×10-6 torr. The reflective layer is evaporated byheating a crucible containing metallic aluminum or silver to the pointof vaporization of the metal. The metal evaporant is allowed to passthrough a stream of energetic electrons thereby partially ionizing metalevaporant. Non-ionized metal vapor proceeds to the primary concentratorsubstrate where it condenses. An electrical field present in the chamber70 accelerates the ionized evaporant toward the primary concentratorsubstrate with this extra energy of acceleration being deposited on thesurface once the ions reach their destination.

Ion bombardment has long been used for improving vapor deposited thinfilms. However, the known processes typically use a beam of ionized gas,atoms or molecules for the bombardment. The beam approach is veryexpensive and not very practical for bombarding such large areas as theparabolic primary mirror. For this reason, we originated a unique systemfor ionizing a portion of the evaporant beam and accelerating it towardsthe target film to achieve the same results. A schematic of the ionizingdeposition system is shown in FIG. 7. The system is made up of asufficiently large vacuum chamber 70 to allow the primary mirror 10 tobe enclosed within the chamber. Both a metal evaporant source 72 and aglass evaporant source 74 are provided and used. Filament 76 is heatedvia an electrical current such that electrons are released from thefilament. Anode 78 is placed opposite the filament and given an electriccharge to attract electrons released from the filament. The electronstream 80 is allowed to pass through the evaporant stream issuing fromeither the metal or glass evaporation source. Typically, an electroncurrent of 100 to 500 mA is sufficient to partially ionize the evaporantexiting the sources. Both ionized and non-ionized evaporant moves towardthe substrate. Plate 82 is located between the sources/filament and thesubstrate. This plate 82 is charged to a high voltage to accelerate theionized evaporant and thereby impart extra energy to the evaporantstream. An additional plate 84 connected to ground, is located betweenthe charge plate 82 and the filament to reduce the number of electronsbeing diverted away from the evaporation source. The substrate 10 iskept at ground potential for safety reasons.

The glass layer 30 is deposited in a similar fashion with the exceptionthat a silicon monoxide material is used for the starting sourcematerial to be evaporated. The silicon monoxide is evaporated from asource while oxygen is injected into the vacuum chamber 70 to create acontrolled oxygen partial pressure in the deposition chamber. The oxygencombines with the silicon monoxide both on its way from the source tothe target surface and at the target surface in order to convert it tosilicon dioxide that forms a stable, transparent fused silica film onthe deposition surface. During the evaporation, an electron streampartially ionizes the silicon monoxide material and the charged plateswithin the chamber accelerate the ionized silicon monoxide towards thesubstrate 10. This added energy from the accelerated ions is depositedat the growing SiO₂ layer on the target surface yielding more mobilityand reactivity that improves the density and adhesion of the glass filmwhile reducing the number of film pinholes, improving the film's weatherresistance. After the glass deposition, the primary mirror 10 is thenremoved from the deposition chamber 70 and inserted into the solarenergy conversion system.

The secondary mirror 14 may also be formed from a sheet of aluminumalloy in much the same manner as the primary mirror 10. That is, a sheetof aluminum alloy may be hot blow formed into a die that is preciselymachined to the desired shape of the specular surface. Then, the convexsurface of the secondary mirror is provided with a polymer layer appliedto the formed aluminum member. Then a reflective metal layer is appliedto the polymer layer and a thin glass layer is formed on the metallayer.

Referring to FIGS. 8-11, the secondary mirror specular surface 16 can bedetermined in the following manner. Firstly, a hyperbolic curve hc isformed which has a geometrical center line axis 40, a concave side, aconvex side, an imaginary focal point IFP intersecting the axis 40 onthe concave side, and a real focal point RFP intersecting the axis 40 onthe convex side. This axis 40 and the hyperbolic curve hc are pivoted inposition about the imaginary focal point IFP in the manner shown by FIG.9. When the hyperbolic curve hc is in this position, there is a point aon the curve which intersects the original position z-z of the axis 40.The segment a-b of the hyperbolic curve hc is then rotated in positionabout the axis z-z to form a surface of revolution that is generallyhyperbolic except that it has been modified to such an extent that thereal focal point RFP now lies on a circle 42 that surrounds the originalaxis z-z. This circle 42 is shown in FIG. 10. Accordingly, the rotatedportion of the tilted hyperbola forms a surface that now focuses lightenergy onto an annular area.

FIG. 10 superimposes FIG. 8 on FIG. 9 and shows the annulus 42 on whichthe solar energy is focused. Accordingly, the surface of revolution hcformed by tilting the hyperbolic curve 90 about the imaginary focalpoint IFP and then rotating the tilted curve about the original axis z-zis used as a basis of making the concave surface of a die against whichthe aluminum alloy sheet is moved by the use of a hot gas. After themetal body is shaped, the reflective surface is smoothed to a specularquality. Again, similar to the primary smoothing technique, a polymerwith sufficient surface tension and low viscosity is applied to thesurface. It is allowed to outgas a sufficient time for the surfacetension to smooth the surface. Care is taken to allow the epoxy to levelwithout running. This may be done by spinning the mirror 14 face down ata rotational speed such that the centrifugal force cancels that ofgravity while the capillary adhesion force holds the film of polymeronto the surface of the part. This technique requires the smoothingpolymer layer to be sufficiently thin to allow capillary adhesion tohold the thin layer upside down without dripping. The adhesion forceneeds to counteract both the component of the gravitational force and acomponent of the centrifugal force. FIG. 12 shows a schematic ofspinning the hyperbolic surface hc; and shows the forces that must bebalanced to maintain the polymer without movement. After the smoothingpolymer layer has cured, the reflective surface is deposited using thetechnique described above for the primary mirror specular surface 12. Ametal reflective layer is first deposited using energetic ions todeposit additional energy. A glass protective layer is deposited next,again using energetic ions of the glass evaporant to supply additionalenergy to the growing film. This completes the fabrication of thesecondary mirror surface 16.

According to an aspect, an annular lens/prism assembly is positioned onthe annular area formed by rotating the real focal point RFP. In FIG.13, the ring of optical elements 50 is shown at the lower end of adownwardly converging annular opening in a member 18. This array oflens/prism elements 50 functions to redirect the ring focus from theprimary and secondary mirrors into a ring of discrete rectangular areas.Each of the rectangles has its long axis oriented along lines radial tothe original focused ring and at equal angles to each other. Theadvantage of this arrangement is that the high intensity, concentratedenergy is now directed only to the active area of the photovoltaic, orother, energy converting elements. See photovoltaic cells PVC in FIG.20. This has the added advantage that interconnecting wires, pathways,and other structures are not subjected to high intensity energy exposureand special design considerations are not required to prevent damage tothose areas.

The lens/prism elements 50 are unique in the way that they split theincoming light beam into two components in order to concentrate it andsplit it into narrow lines. The optical axis of each lens/prism element50 is a perpendicular line extending vertically through the center ofthe elements 50 from top to bottom. In the radial dimension the elements50 act as prisms with the geometry shown in FIG. 14-17. The elements 50act as prisms to rays incident at off-axis angles in the radialdirection that penetrate into the element and are reflected from theflat ends of the elements 50 by total internal reflection. The opticalperformance of the prism is illustrated by the ray traces in FIGS. 15and 16. The elements 50 act as double-convex lenses to rays incident atoff-axis angles in the circumferential direction. FIG. 16 illustratesthe geometry of the double-convex lenses. The optical performance of thelenses is shown by the ray traces in FIG. 16. The circumferentiallyoff-axis rays that penetrate into the element 50 are refracted to afocus beneath the element 50. The thickness of the double-convex lenscan be varied to optimize their performance in a good design. Forexample, the simplest form is when the two convex surfaces form acomplete circle or cylindrical lens. In this case, for most practicalglasses, the beam is focused at some distance between the lower surfaceof the lens. If the thickness of the lens is increased, the rays can bemade to focus inside the glass material. In the example shown by FIG.16, the rays are focused at the lower surfaces of the lenses. In thisway the performance of the lenses can be optimized for a particularsystem. By the unique combination of the prism and lens functions, thecontinuous ring of concentrated sunlight on the top surfaces of thelenses is formed into narrow rectangular focal spots formed just beneaththe lenses. Finally, the sides of the lens/prism elements 50 are taperedas shown in FIG. 17, so that they can be fitted into an annular array asshown in FIG. 14.

By breaking the incident light rays into two components, one radial andone circumferentially, the spread of angles that the prisms and thelenses individually must operate on is limited to a much smaller rangeand allows pointing error tolerance for the system even at the very-highconcentration levels of the CHP system.

FIG. 18 is a schematic of the electrical system. The particular systemthat is illustrated includes four solar energy collection systems I, II,III, IV connected in parallel to electrical conduits leading to acustomer hookup CH. Each of the four collector systems I, II, III, IVinclude an array of twenty-four series connected/parallel GaAsPhotovoltaic Cells which are adapted to generate 26 volts at no load and24 volts at 10 amps. The circuit includes eight 15-amp circuit breakersB and a ground wire GW. Positive and negative conductors lead from thecircuit breakers to a user's electrical load. The system includes amicrocomputer for tracking drive and control and a small storage batteryfor startup.

The photovoltaic cells PVC serve a dual function in the system. Theyabsorb the concentrated sunlight and converts a portion of it toelectricity and a portion to heat, or thermal energy. Thus, it serves asboth an electricity generator and a heat generator in the system. Inorder to accomplish these two roles efficiently, the photovoltaic cellsPVC must be fabricated from semiconductor materials with sufficientlywide band gap to maintain efficient electrical conversion at relativelyhigh temperatures. In general, the wider the band gap of thesemiconductor material, the less the photovoltaic cells efficiency willbe degraded with rising temperature. Thus, a tradeoff is required forthe application being considered depending on the relative importance ofelectricity production and heat production. For example, if a GaAsPhotovoltaic Cell PVC is used, then the system can produce heat attemperatures up to about 100° C. while still maintaining an electricalconversion efficiency that is approximately 95% of its efficiency at 25°C. Thus, the system lends itself well to wide band gap photovoltaiccells having both single and multiple tandem junctions. The inventorshave determined that, the cost of photovoltaic cells made from wide bandgap materials and in multi-junction configurations has precluded theiruse in terrestrial applications. For this reason, the very-highintensity produced by the concentrator system disclosed herein allowsthe use of small, economic area of photovoltaic cells. For photovoltaiccells PVC to operate efficiently at very-high light intensities, aunique design is required. FIG. 19 illustrates a very-high intensitysolar cell design. The cells PVC have long and narrow active areas fortwo reasons. First, it corresponds to the illumination pattern providedby the tertiary concentrator lenses 50 shown in FIG. 14. Secondly, itprovides a very short path length for conducting the photo-generatedcurrent away from the cells PVC. Photovoltaic cells PVC under high lightconcentration generate very large currents at low voltage; therefore,any series resistance in the cell would drop the voltage and, in turn,the efficiency of the cells. The current from the high-intensity cellsis collected and conducted off of the cells by means of a pattern ofelectrically conducting metal grids G overlaying the active areas of thecell. The series resistance in the grids G is proportional to the lengthof the grids. For this reason, the long narrow cell design with itselectrical bus bar BB running parallel to the long dimension of thecells PVC permits the necessary short conductor grids G.

Another constraint on the grid lines G is that they cover as small aportion of the active area as possible so as to not shadow the activearea of the cell PVC and prevent light from entering the cell PVC. Theelectrical resistance of the grids G is also proportional to their widthand height. For this reason, making the cells PVC tapered, getting wideras they approach the bus bar and the current from the cells PVCincreasing can help to optimize the low resistance with the need for lowshadowing. Making the grids G as thick as possible can also reduce therequired width of the grids G. To those skilled in the art, many othergrid-bus bar designs for minimizing resistance and maximizing activearea will be apparent. Typical metals for use in the grids and bus barsare gold, silver or copper.

The photovoltaic cells PVC are mounted on a special, electricallyinsulating, thermally conducting substrate 120. The substrate 120consists of a thin alumina sheet 122 with gold plated copper cladding124 bonded to each side. At the top side of the substrate, the cladding124 is etched to form a circuit pattern for making electricalinterconnects 126 between the photovoltaic cells PVC that are bonded tothe etched copper circuit pattern as shown in FIG. 20. In seriesconnected strings, photovoltaic cells PVC are subject to damage due toreverse bias breakdown if part of the string is being illuminated due toa partial shading of the concentrator. Every two photovoltaic cells PVCin series string of twenty-four cells are protected from reverse biasbreakdown by a diode D connected in opposite polarity to the twophotovoltaic cells PVC.

FIG. 21 illustrates the electrical connection of the photovoltaic celland diode arrays. The electrical power is conducted off the array by twocopper leads attached at the center of the annular board as shown inFIG. 20. The cells PVC are cooled by a stream of circulated water whichcontacts the back of the gold plated copper cladding on the back of theceramic circuit board. In this way the thermal energy is extracted fromthe cells and delivered to a heat exchanger located at the base of theconcentrator.

FIG. 22 illustrates the overall thermal collection and control system.

FIG. 22 shows the secondary mirror 14 being cooled by a liquid whichflows from a circulation pump 130 to an “in” conduit 132 that leads intothe base of the concentrator 18 and from it through the post P into thehollow interior of the secondary mirror 14. The heated coolant flows outfrom the hollow interior of the secondary mirror 14 through an “out”conduit 134 which extends back through the post P and onto a two-wayselector solenoid valve 136. If the thermal energy is to be delivered toan external load, then the valve 136 is operated to deliver the heatedcoolant to a liquid-to-liquid heat exchanger 138. The thermal energy istransferred to the user's fluid for transport to the desired thermalload. If the external thermal load does not require any portion of thethermal energy, then the valve 136 is operated to direct the coolant toa liquid-to-air heat exchanger 140 where it is transferred to theambient air. Thermocouple sensors TS mounted on the substrate for thephotovoltaic cells PVC monitor the temperature of the photovoltaic cellsPVC. If the cell temperature rises before a preset threshold, then theconcentrator array is directed by the onboard microcomputer to track thesolar energy collector(s) away from the sun in order to prevent damageto the system. As a backup against computer failure, a thermal switch Sis mounted on the housing of the receiver base. This normally openthermal switch S is connected in parallel with the vertical drive gearmotor. If the temperature rises too high, the switch S closes andprovides continuous dc power to the motor with the proper polarity todrive the concentrator array to a ground facing position until thethermal problems can be analyzed and corrected.

FIGS. 23-28 relate to the control system composed of the elements markedCS in FIG. 3. This system comprises a sun sensor that supplies sunposition signals to a microcomputer that processes the information andsends control signals to gear motors that drive the concentrators andholds them locked onto the sun to an accuracy of +/−0.1°. Themicrocomputer also monitors water temperature and adjusts the flow ratein the coolant loop to achieve the desired operated temperature. Themicrocomputer further serves to shut the system down at night andposition the concentrator dishes to face the ground, wake the system upin the morning and acquire the sun, monitor the photovoltaic celltemperatures and drive the concentrators off sun if the cells overheat,monitor wind speed and rotate the concentrator dishes to face down(edge-on to the wind) if wind speed exceeds at threshold amount. Themajor functions of the sensor and control system are described ingreater detail below.

The CHP solar concentrator system requires several specific functionsfrom its sun sensing system. Firstly, the tracking system used two-axisactive closed loop continuous sensing of the sun and does not depend ona clock and timing algorithm for finding and tracking the sun. Since ituses no timing algorithm, the sensor senses the sun's position fromanywhere in a 180° solid angle, or hemisphere. Once the sun is found,the sun position sensor SPS provides sufficiently sensitive signals toallow pointing at the sun with +/−0.1° of accuracy. The sensor alsodiscriminates between direct solar illumination and the light frombright cloud rims and other interference, which may be due to strayreflected light or ground based light sources. Finally, the sensor andtracking system may be economical enough for commercial sales.Heretofore, no system existed that met all of the above criteria.

The sensor SPS is based on a different physical phenomenon than that ofconventional sensors. The sensor SPS lends itself better to protectionfrom stray light and provides more sensitive detection of small angularoffsets in pointing accuracy. Most of the available sensors may use somesort of tall column with photo sensors attached to its four sides. Asthe column points directly at the sun, the cells are turned edge on tothe sun and no signal is produced. As they turn away from the sun, thecells on one side of the column are illuminated yielding a signal whilethe cells on the opposite side are shaded and yield no signal. Anothertype of sensor utilizes cells mounted flat in the bottom of acollimating tube. The trouble with these designs is that the signalsproduced are highly non-linear and sensors are either very prone toelimination by stray light (columnar type) or they are not amenable tosensing the sun at large error angles (collimating tube type).

The sensor SPS is based on non-linear dependence to the angle ofincidence of reflectance and transmittance of light at the surface ofphotovoltaic cells and in dielectric materials such as glass in order toovercome the difficulties described above. FIG. 23 shows a typicalreflectance curve for glass as a function of the angle of incidence ofincoming light. FIG. 24 shows the same effect manifested in thetransmittance of light into a photovoltaic cell. The current generatedin a photovoltaic cell is directly proportional to the amount of lighttransmitted into it. To generate the data presented in FIG. 24, a glasscovered photovoltaic cell was placed beneath the beam of collimatedlight and rotated through one hundred and eighty degrees (180°),beginning at −90° (edge-on to the light beam facing in one direction)through 0° (normal incidence) to +90° (edge-on facing in the oppositedirection). As can be seen, the signal produced is non-linear with therate of change being much greater at larger angles off normal than itsangles off-axis. This non-linear cell current response with angle ofincidence can be used to generate a sun sensor that is highly sensitiveto angular error from normal incidence and produces a linear signal thatincreases with increasing off-axis error angle.

FIGS. 25 and 26 show a physical embodiment of the sun sensor. Small,glass covered photovoltaic cells 150 are mounted on the angled sides152, 154, 156, 158 of a truncated pyramid 160. When the sensor istracking “on-sun”, the sun is aligned directly over the truncatedpyramid 152, 154, 156, 158 of the sensor whose axis is defined by avertical line passing through the center of the pyramid. This line islabeled optical axis in FIG. 26. As the sensors tilt away from the sun,the angle of incident light hitting the glass-colored sensors 150 ischanged, becoming steeper on the side away from the sun and less steepon the side nearest the sun. If the side angles are chosen so that thecells are operating near the on-set of non-linear reflectance (about60°) then the light entering the sun-facing cells increases rapidlywhile the light entering the cell facing away from the sun decreasesrapidly. The angles of the sides are optimized to yield the maximum andmost linear error signal. FIG. 28 illustrates the typically producederror signal. The signal has excellent magnitude, is very sensitive tosmall angular changes, is very linear over a large range of angles aboutnormal, and increases in magnitude proportional to the angle off normal.

The non-linear reflectance based sun sensor is extremely sensitive tosmall angular errors about its normal which allows it to hold theconcentrator locked on sun to a very tight tolerance, +/−0.1° beingtypical. The low profile of the sensor enables the use of a shadowshield ss to protect the sensor from stray reflected light or locallight sources. FIG. 27 shows an axial sectional view of a typical shieldembodiment. The fine sensor assembly is located at the bottom of acylindrical shadow shield ss. The shield ss limits the sensor to afairly narrow cone of entrance angles so that it becomes the “fine”sensor of the system. For coarse sensing, four photovoltaic cells 162are located on the outside of the shadow shield ss. These four cells arearrayed ninety degrees (90°) apart around the circumference of theshadow shield. This configuration provides two opposing pairs of cellsfor north-south and east-west sensing respectively. In case the sun islocated behind the concentrator array, there are also two rear facingphotovoltaic cell sensors RFC.

The signals from the fine sensors, coarse sensors, and the rear-facingsensor are processed by the control microcomputer. If there is little orno signal on the fine and coarse sensors but there is signal on therear-facing sensor, then the computer directs the gear motors to drivethe system, westward until the coarse sensors pick up signal. At thispoint, the computer transfers control to the coarse signals and theconcentrator drive system will seek the sun in both horizontal andelevation directions. When the computer detects sufficient signal fromthe fine sensors, then it transfers control to the sensors for lockingon-sun and maintaining the desired tracking tolerances in each axis. Inthis way, a highly sensitive, highly accurate tracking system that isimmune to stray light signals is achieved. The remaining problem “cloudchasing” due to scattered light from bright cloud rims is solved bysoftware. The computer is instructed to ignore signals that are not upto a threshold value typical of direct sunlight.

The onboard computer is the heart of the control system. In addition tohandling the tracking signals and controlling the tracking motors, thecomputer handles a variety of other signals from its internal clock,limit switches, thermocouples, and a wind sensor to keep the systemoperating safely and efficiently. The limit switches are located in themotor drive trains and are tripped when the motion of the concentratorreaches the extreme limits of travel in either direction and in bothaxis. For example, at the end of the day, the horizontal drive systemrotates from its westerly, sundown position back to the east. It tripsthe east limit switch locating the concentrator for beginning the day'stravel the next morning. Usually (except at summer solstice) in theevening and the system does not drive to the extreme westerly positionso an internal clock is used to signal the computer to drive back to itseasterly “home” position at a program time when energy collection isfinished for the day. Similarly, at the program time in the evening,after the system reaches its easterly “home” position, the computerdrives the concentrator to face down. The computer also samples signalsfrom a wind sensor and, if the wind velocity exceeds a programmedthreshold, then the computer directs the concentrators to theground-facing position, i.e. horizontal to the wind until the windsignal drops below the threshold value.

The computer also monitors a thermocouple located on each photovoltaiccell array and if an array indicates a temperature exceeding aprogrammed threshold, then the concentrator is driven to theearth-facing position until the problem can be evaluated and correctedas necessary. As a backup, in case the computer malfunctions, a thermalmechanical switch is located on the photovoltaic receiver body. If thereceiver body reaches a high enough temperature to trip thethermal-mechanical switch, then it overrides the computer and suppliespower directly to the elevation drive motor to drive the concentratorsto the ground-facing position.

For normal operation, the computer monitors the coolant fluidtemperature and controls the pump flow rate to adjust the fluidtemperature to a program value. If the temperature is rising and thefluid flow rate is at its maximum, then it is assumed that the externalthermal load cannot accept all the thermal energy being generated andthe fluid flow is diverted by a solenoid valve to the liquid-to-air heatexchanger to control the fluid temperature.

In another embodiment, the photovoltaic cell array can be replaced by alight absorber to absorb the concentrated sunlight and convert itdirectly to heat and transfer it to a desired application. The desiredapplication can vary from domestic hot water, water purification,commercial processing, or absorption air conditioning. The heat can alsobe used directly to (1) drive heat engines such as Stirling engines, (2)super heat steam to drive a steam engine or turbine, (3) to fuel athermal electric generator, or (4) drive any other type of thermalengine or heat application.

It is within the scope of the various embodiments to direct a laser beaminto the parabolic dish in a direction parallel to its optical axis andthen use photovoltaic cells that are sensitive to the laser photons toconvert the laser beam to electrical power. In this manner, power can betransmitted over long distances without the use of electrical wire. Itis also within the scope of the various embodiments to direct amodulated laser beam into the concentrator system parallel to itsoptical axis and use a detector sensitive to the laser photons to detectand analyze the modulated signal. In this way the system can be used asthe receiver in a laser transmitted communication system. Herein, “lightenergy” is generic to sunlight (soar energy), laser beams and otherlight beams.

The various embodiments that have been illustrated and/or described areonly examples and, therefore, are non-limitive. It is to be understoodthat many changes in the particular structure, materials and featuresmay be made without departing from the spirit and scope of the presentdisclosure. Therefore, it is our intention that the patent rights not belimited to the particular embodiments that are illustrated and describedherein, but rather are to be determined by the following claims,interpreted according to accepted doctrines of patent claiminterpretation, including use of the doctrine of equivalents.

1. A light energy collection system, comprising: a primary mirror havinga concave specular surface constructed and positioned to receive lightenergy and focus it towards a focal point; and a secondary mirror havinga convex specular surface constructed and positioned to receive focusedlight energy from the primary mirror and focus it onto an annularreceiver positioned between the primary mirror and the secondary mirror,wherein the annular receiver includes an annular array of opticalelements operable to focus the light energy received from the secondarymirror onto a ring of discrete areas within the annular receiver.
 2. Thelight energy collection system of claim 1, wherein a ring oflight-to-electrical conversion units are positioned on the annular ringof discrete areas.
 3. The light energy collection system of claim 1,wherein the concave specular surface of the primary mirror issubstantially parabolic.
 4. The light energy collection system of claim1, wherein the convex specular surface of the secondary mirror isgenerally hyperbolic.
 5. The light energy collection system of claim 1,wherein the optical elements include lenses.
 6. The light energycollection system of claim 1, wherein the optical elements includeprisms.
 7. The light energy collection system of claim 1, wherein theannular receiver is a body adopted to convert the light energy to heat.8. The light energy collection system of claim 1, wherein the annularreceiver is adapted to convert the light energy to electricity.
 9. Thelight energy collection system of claim 2, wherein thelight-to-electrical conversion units include photovoltaic cells.
 10. Thelight energy collection system of claim 2, wherein thelight-to-electrical conversion units are connected together.
 11. A lightenergy collection system, comprising: a primary mirror having a concavespecular surface constructed and positioned to receive light energy andfocus it towards a focal point; a secondary mirror having a convexspecular surface constructed and positioned to receive focused lightenergy from the primary mirror and focus it onto an annular receiverpositioned between the primary mirror and the secondary mirror, whereinthe annular receiver includes an annular array of optical elementsoperable to focus the light energy received from the secondary mirroronto a ring of discrete areas within the annular receiver, said convexspecular surface being formed by: providing a hyperbolic curve formedsymmetrically about an axis and having a concave side, a convex side, animaginary focus point on the concave side and a real focal point on theconvex side; tilting the axis and the hyperbolic curve about theimaginary focus point, so that in its tilted position the axis isseparated from its original position by an acute angle and the paraboliccurve is tilted from its original position; rotating the portion of thetilted hyperbolic curve that extends from the original axis to thetilted axis and beyond the tilted axis, about the original axis, so thatthe rotated portion of the tilted hyperbolic curve forms a surface ofrevolution about the original axis; and providing the convex specularsurface of the secondary mirror with the shape of the surface ofrevolution so that the specular surface will have an annular focuspattern.
 12. The light energy collection system of claim 11, furthercomprising an annular array of lens/prism elements positioned at theannular focus, said elements each having a convex top, a convex bottom,planar inner and outer ends, and planar sides which extend radially sothat the lens/prism elements substantially abut to form an array ofsubstantially annular form.